1. Field of Invention
The present invention relates to the field of testing digital logic devices. More particularly, the present invention relates to a system for determining the dynamic threshold of the digital logic device being tested. More particular yet, the present invention involves determining the dynamic threshold of the digital logic device through an iterative procedure, where the procedure is substantially automated.
2. Description of Prior Art
While digital logic devices are designed with their operational characteristics theoretically determined, there is always the requirement of verifying actual operational characteristics due to known or unknown manufacturing variables such as substrate contamination or inherent impurities that ultimately affect device operation. The ways in which operation suffers include a decrease in a device's tolerance to input extremes and fluctuations. This results in increased failure situations such that the device's dynamic output as shown by its alternating current (AC) characterization will reveal propagation walk, glitches, forced highs and lows, and the like. The point at which these failure situations occur is termed dynamic threshold and can be determined through testing by an electronics technician. Therefore, testing of the given digital logic device is necessary so as to provide product designers with a degree of predictability with respect to the device's characteristics and with respect to the device's compatibility with other elements in the product within which the device is used. In order to obtain an AC characterization of a digital logic device through propagation-delay tests, setup-and-hold tests, functional-speed tests, access-time tests, refresh-and-pause-time tests, and/or rise-and-fall-time tests, it is usually in the particular test specification to run dynamic threshold tests. Ideally, dynamic threshold is reached when the input voltage to the device is either raised or lowered to the point where the output of the device reaches a critical state of instability known as metastability where a failure condition may occur in any of the aforementioned forms.
The conventional manner by which dynamic threshold tests are administered is manually with the aid of an oscilloscope. The oscilloscope is monitored visually by a technician while the input voltage to the device is manually swept across a range of voltage values. When the technician sees the output of the device fail--i.e., exhibit one of the failure conditions--on the oscilloscope, the technician then records the corresponding input voltage at that moment. Because the failure mechanism when the output goes into metastability is complex in nature, requisite human detection during such conventional testing cannot be avoided. Dynamic threshold output failure is a complex condition that may take many different forms as mentioned above. Further, no oscilloscope is capable of detecting such a specialized and varying condition as is the dynamic threshold condition. Thus, human detection, while costly and time-consuming, has been the necessary standard.
In addition to the high cost and burdensome amounts of time required, human detection is subject to the vagaries of interpretation. As already mentioned, dynamic threshold conditions may take many different forms. The difficulty in visually interpreting an oscilloscope in order to determine dynamic threshold may be exacerbated by the technician's degree of familiarity with a given oscilloscope or even exacerbated by simply choosing an inappropriate scale to view a particular test. Further, human detection is limited to a real-time viewing that can result in outputs that are exceedingly difficult to reproduce. Although many oscilloscopes can store an output waveform, metastability must first be detected by the technician before such storage can occur and even then analysis is by visual detection and limited to the single-stored waveform.
Accordingly, the prior art fails to provide any method or device that can provide cost-effective, reproducible, and automated dynamic threshold testing. Therefore, what is needed is a system that provides a dynamic threshold testing procedure without high costs in both technician-time and money. What is also needed is such a system that can easily utilize existing oscilloscopes without requiring labor-intensive human detection of dynamic threshold conditions. Still further, what is needed is such a system that provides both automated and easily-reproducible testing.